I have a friend who has an older longbow that was made by somebody else. However, he is wanting me to get him two new strings. I have one of his old strings to get length from. So what type of string material would be safe for the bow?

Without seeing the limb tips and what they are made of, he'd definitely be safe with B-50.

Try the new B-55. Its Dacron so its safe to use on older bows but it doesn't have all the B-50's creep.

Who can I get to make a string out of B 50

There are three safe options, B-50, B-55, and B-500.  B-50 and B-500 are the same stuff from two different vendors. B-55 is the newest form and it is slightly better.

There should be LOTS of places you can get the string made or even find one in stock.  We have quite a few in stock at Lost Nation Archery and we can make anything in 2 days, provided we are not too busy to get to it.  Currently we are saying 2-3 days.  You will find other vendors who are sponsors here.

How old a bow??
Is it a deep cored longbow? Im shooting 18-26 year old JD Berry bows with 14strand ff+ and they handle them well , but these are deep cored bows