
I'm afraid that if your friend hasn't looked into it much she may well be shocked at how many "bad" points there are!
You don't say what breed the dog is but every breed has standard health checks for inherited problems, for example x-rays for hip dysplasia, a variety of different eye tests, blood tests, to be honest without knowing the breed there could be so many more. She would need to contact the national Breed Club and get the full list of requirements for her specific breed. In order for your friend's dog to be of interest to a responsible bitch owner he would have to have the veterinary certificates proving he had passed all tests relevant to his breed, and some of these require to be carried out annually. NB, all these tests cost a lot of money up front!
Even supposing he sails through them all, she may well never recoup the costs of tests alone! Stud dog owners don't advertise for bitches - it is the owners of the bitches who research what they are looking for in the stud dog and they who approach the owner of the males - so he would need to have a pedigree that was of interest to serious breeders, be a physically high quality representative of his Breed Standard, excel in temperament and be entered in the competitive or working disciplines of his breed in order to be noticed and inspected by potential brood owners. Again, "campaigning" him - basically having him in the right places in front of the right people ate the right time for visual assessment - costs an awful lot of money too.
Supposing all that goes to plan for your friend, from a purely practical point of view stud dog owners have to be very accommodating as bitches' seasons don't always go by the calendar. She could have a sudden phone call to say the bitch is en route much earlier than expected and when it really is inconvenient (bitches always visit the dog by the way!); Your friend needs to be prepared to "put up" the bitch for several days if she has been brought from some distance away and the time is not right. Has she VERY secure facilities for housing a stranger's precious bitch? Does she know how to assist in the actual mating process? How will she cope with her dog's lack of appetite if he has a resident in season bitch tantalisingly out of reach for a week or more and has only one thing on his mind?
Stud dog owners should also have a life long responsibility in conjunction with the owner of the dam for ensuring that puppies sired by their dogs have good homes and can be re-homed or temporarily fostered by them if in the future the puppies are unwanted by their original owners. Quite honestly there are a zillion negatives as your friend is so totally inexperienced and unprepared and yes, among those are the fact that it could change his behaviour in a variety of ways - some not too difficult to live with but potentially others that could make life pretty miserable for her and the dog :o(
As to "do they need a constant supply of females" - no - but it's tricky telling the dog that if the owner doesn't have more general knowledge about the whole breeding game to start with :D
Now for the good points - nope, sorry, it's too late ( or is that early???) for me to come up with any :-(
The plus point of NOT *studding* him is she'll save an awful lot of money, stress, travelling "to be seen" and potential straying and/or temperament problems later with her dog.
I hope this gives your friend some food for thought - none of the above is intended to offend you for asking on your friend's behalf or ridicule your friend for thinking about her dog's potential as a stud. It's a commonly raised question on forums such as this simply because few people outwith the breeding and competitive world of pedigree dogs actually know just how much is involved. ;-)
Teri