What height should a climber consider safe on a boulder such that in case of a fall her/his spotter can guide her/him fall to safely land on feet?
Here, safety is for both of them, and not just the climber.
Is there a guideline that climbing/bouldering folks follow?
This varies a lot so it's hard to write a definitive answer. But if you're bouldering (full stop) and you think you're going to fall off (high in your grade) and your concerned about your landing, you want a spotter.
Height is pretty much irrelevant.
The spotters jobs include:
This is just as relevant at 1m as it is at 6m.
Remember a spotters job is not to catch you. They are there to ensure you land safely on the mats. This may mean they push you as you fall or flip you onto your feet but they should never try and actually catch you; this will likely just result in both the climber and spotter getting hurt.
This doesn't mean you always need a spotter; they just add an extra dimension of safety. It's up to you to decide if you feel you want one or not. It's likely advisable if the landing is bad (not flat and/or covered in rocks, etc) or you're really pushing your grade and may have an uncontrolled fall.
We seem to be getting into the realms of when does bouldering stop being bouldering and start free soloing. TBH I don't know, I don't think anyone does. It really depends on your own confidence. Personally I don't think I've climbed over 3m or so without a rope. If you're brave enough to climb higher than this with just mats then good luck. A spotter may help.
Here's some advice form Climbing.com on spotting on high ball problems (emphasis my own):
Never assume you’re being spotted and don’t be afraid to request one. As climbers, we’re responsible for our own safety. Spotting a super-highball can be dangerous, and the main goal for a spotter is to guide the falling climber onto the pads, not to catch her or slow her fall. Discuss the landing zone, how you might fall, and where the spotters should stand. As the climber falls, the spotter should aim to grasp her hips and fall with her toward the pads. With really big highballs, use a “floating pad” to cushion the impact: Two people hold a regular pad a few feet above the main pad layout, tracking underneath the climber and letting go when the climber hits this pad. The climber will hit the floating pad first, slowing her fall, before landing on the layered pads below.
While spotting, keep fingers and thumbs together in a cupped position (spoons) as opposed to spread out (forks). This prevents finger and hand injury. With arms up, keep your elbows and knees slightly bent.
It's very subjective and you really need to make you own decision here but a spotter will help move the mats into the correct position if nothing else.
I think it depends on the situation, what's on the ground, whether or not the boulder is overhanging (chance of falling headfirst), traverses etc. But personally as a spotter I'd feel pretty redundant if the climber is over 6m (approx 20 feet) off the deck. Then my main concerns would be: protruding rocks, overhangs and trying to keep their head off the floor. Then again, I hardly ever boulder with more than 2/3 pads and friends, so with 20 pads and mates, maybe it feels different.
This will vary for different people, I feel a lot more confident with a strong spotter than some of my scrawnier friends.
Depends entirely on the skill level of the climber and the spotter.
Falling is a skill, and catching a fall is a skill. The more skilled the climbers are, the higher risks they can take. If either one is uncomfortable with how high a boulder is, then the problem shouldn't be attempted.
Catching highballers usually requires a lot of crashpads, and a lot of spotters, and whomever is spotting needs to be alert and make sure the pads move to where they need to be as a climber falls. The climber must also be certain that if they take a fall, that it's controlled so they can land on their feet and take as much of the force of from the fall as possible with their legs; this often requires them to bail off before they blow off. When spotting high falls, you're priority is to ensure the pads are under the climber, and that the climber hits the pads by directing their fall if they fall outside the LZ, sometimes injuries can't be prevented on highballers, climbers attempting them acknowledge this risk, and do their best to come out of a fall with as little injury as possible.
Watch the spotters as they spot Alex Hannold on "Too Big To Flail".
You should always look to guide the climber to the crash pads (mats) and try to let them land on their feet only if possible rather than catching them. But when it comes to it being high, you should adjust the mat(s) rather than just yourself to catch/redirect them.
