I don't know a lot about this, but I was wondering if this problem wasn't caused by a nutritional deficiency since the cat had been a stray. Also, it is so wonderful that both you and your friend were willing to help this cat out.
There are many wonderful people here who are very knowledgeable and may be able to give you more help.
I noticed shortly after we got Danielle from the shelter, that she had one dry cracked paw. It did not seem to bother her (I did not see her licking it). Eventually, it scabbed over, and fell off, to reveal new pink skin underneath. She is fine there.
Perhaps it is nutritional, cause I know that Danielle was fed crappy food in the shelter.
The only time I have ever seen it was on outdoor only cats. From all the climbing and weather, they get dry and cracked. If they seem to bother him you can put some bag balm on them.