If they are neutered, then yes, they can be housed together. If they are not, then they need to be neutered; once the unneutered ferret comes into rut, he will become aggressive toward the other male ferret. If the original ferret is much older than the new one, I would keep them separate until the little one gets big enough to hold his own during play. I would honestly suggest keeping them separate anyway for at least two weeks just in case the baby has anything contagious like ECE he could pass on to the older one. Make sure they take care to thoroughly wash their hands(and anything that the two ferrets share such as toys, etc.) between handling the individual ferrets.
If the older ferret has been an "only" ferret all his life, you may want to introduce them gradually and not make them live together right away. Doing so may cause the older one stress. If they are in separate cages, once the quarantine period of 2 weeks is over(and both ferrets are not showing any signs of illness), you can switch their bedding, so they get used to one another's smell. When let out to play, always supervise. If one or both seems to be very stressed during or after these interactions, put them back in their cages.
In the beginning, your neighbor may see a lot of really rough, almost violent "fighting". This is play for ferrets and perfectly normal. They may bite each other, drag one another around by the scruff of the neck, hiss, mounting and pretty much sound like they are killing one another. They are establishing a pecking order and deciding who is boss. If they both want to be boss, the fighting can go on for weeks or even months. If one is allowing the other to drag them around, this is a sign of submission--they're pretty much saying "I give up". Or both of them may not care either way who is boss and may get along right away with no displays of dominance whatsoever. If there is any blood drawn, separate them immediately. Also, if one is grabbing the other around the throat, this is a sign of aggression and not part of the normal dominance play. Separate them and try again another day.
Watch for any changes in appetite or behavior of both of them, whether they are living together or separately. Loss of appetite, diarrhea, vomiting, lethargy..are all signs of stress in a ferret. They can be signs of other conditions, but if they present themselves after the introduction of a new ferret and disipate upon the separation of the two, then it's probably stress. Make sure the little guy isn't getting bullied in the cage by the older one too much. If he won't let the little one eat out of the same bowl as him(beats him up or barks and bites him when the little one goes near the bowl) or won't allow him to sleep in the same hammock (or even near him), then this means there's a problem. This doesn't happen often, but if it does, separate them temporarily OR provide extra bedding and a food bowl for the little one.
Integrating ferrets can go several ways--they'll either get along immediately, get along eventually after some dominance play or never get along. Your neighbor will just have to supervise and see how it goes.
If the little one came from a pet shop, keep an eye out for signs of ECE(Epizootic Catarrhal Enteritis; symptoms:green diarrhea, vomiting, loss of appetite, etc). This is a highly contagious virus(only ferrets get it) and is frequently passed on by new kits being introduced into a household.