It is not as common but neutered males can mount and tie with mounted females. Puppies should not result but dogs can transmit diseases this way and it is rude so you should take immediate steps to strongly discourage this behaviour. In your case, since you are not sure if the male is indeed neutered and you are not sure if you can recognize a heat in your girl (it's not always obvious) my suggestion is you keep them apart.
Cryptorchid dogs are not necessarily sterile.
As said above mounting is not always sexual in nature. Some times it is dominance and it can just be something the dog enjoys. Both boys and girls, neutered or not, can mount and hump. It is not an action reserved for only intact boys.
You need a Vet who can tell you for sure if the boy is neutered. If he is retaining both testicles then he needs to be neutered. He's only 5 months old, sometimes they will drop late but if they don't he is at increased risk of some ailments especially cancer. Your Vet should be able to palpate and find the testicles, if they are retained.
As for "fixing" either the boy or the girl, yes the traditional age has been about 6 months. More and more though, as we learn from reseach into the risks as well as the benefits of altering, advice is being given to wait till more like a year or even 2 or 3 years in the bigger breeds. With the situation in your household you may not have this luxury but here is some reading that may shed some light on why more and more of us are waiting till later. Some breeders will build the requirement to wait till later and allow the dog to mature physically into their health guarantees. If your own girly pup is from a breeder you should check your health guarantee.
http://www.naiaonline.org/pdfs/LongT...uterInDogs.pdf
http://avmajournals.avma.org/doi/pdf...ma.231.11.1665
The two links above give a lot of information on the risks and benefits of altering your boy and girl dog. If you are interested in reading more PM me for an extensive list of other articles on the subject.
Good luck with your foster and your own little girl pup.