I'm pretty sure that's illegal in most of western Europe. In theory at least. You can't just go and help a friend paint their house, but you can go and paint a child or parents house. It's pretty much impossible to enforce in practice. If you do voluntary work through an NGO it's different though, then it's not counted as illicit work.
It likely depends. The point of laws like this is that they don't want people to exchange favours as a means to avoid paying taxes. When you help a friend paint his house you usually expect that he returns the favor and that's not allowed. If there's no expectation to return the favor it's fine. Anyway, these things are hard to prove.
It's important for a mutual friendship though. If you're there for your friends when you need them, you kind of expect for them to return the favor. Friendships are usually not one-sided, that particular "honor" is more common in family relationships.
