Should I bond my rabbit with a friend's rabbit?
You should not bond rabbits unless you plan on having them together indefinitely. Otherwise, there is no reason to have them together. Casual playmates aren't a thing for most rabbits, and it's either hate or a long road that leads to a very strong relationship. It would be better to get them an actual new friend from a shelter instead of putting them through all that with a bun they will not see sometime unexpectedly in the future. Rabbits should pick out a companion themselves, then form a lasting bond and never be alone. 
Can I have my rabbit play with my friend's rabbit?
In small groups, rabbits will fight or bond tightly. Unlike dogs, rabbits are very territorial and do not get along easily. If they fight, it could end up in expensive medical bills. If they bond, it will be cruel to separate them. As a result, playdates for rabbits are not a good idea.
However, social playdates can be possible with a group of 7 or more spayed and neutered rabbits on neutral territory. The San Diego HRS, Minnesota Companion Rabbit Society, and several other rescues often host these "hoppy hours" under experienced supervision.
My two bunnies used to be happily bonded, but now they are constantly fighting. What happened?
If one or both of your rabbits were unfixed and young, chances are that one or both are going through sexual maturity. Territorial feelings and aggression increases once bunnies start producing their hormones. Your rabbits may be fighting to reestablish dominance through humping and chasing. The easiest way to fix such behavior is to spay and neuter your rabbits. You may need to work on re-bonding the rabbits afterwards, though, since scents change when rabbits go through sexual maturity. Wait a month after the operation to make sure all hormones have dissipated before doing so.
If your two buns are different sexes, it is vitally important to separate them now before the female gets pregnant. In fact, you may find out that your rabbits are a different sex than you thought when taking them in to the vet to be fixed. If you got your rabbit from a pet store, it was most likely improperly sexed, as it very difficult for anyone but an expert to tell.
How do I bond a new bunny to my cat/dog? or vice versa?
Please see the article Relationships with rabbits#Other Animals.