I have been in some rest room stalls that are big enough to change in, and some that have hooks, but I would say the majority of fast food rest rooms are either really, really not clean, and/or very small and/or very unlikely to have a hook. Its also not uncommon to have to purchase something in order to get an employee to unlock it for you, so its not free. And that's assuming there is a fast food place along the way from work to the funeral home - and that you know where it is (if the funeral home is in a different neighborhood then you live in for example).
Since you say the friend told you nicely, I'm not going to fault them. You obviously felt they were doing it out of friendship and for future reference. I can see a good friend saying "just so you are aware, jeans aren't normally worn to viewings."
I agree it would have been better to find away to change into a pair of slacks if possible, but I really doubt the family noticed.
I don't even buy this excuse. Unless the OP is blind, by that time she would have already seen what other people were wearing and surely would have noticed everyone else was more formal and would then know for next time.
to the departed, and the departed them-self come into play as well. I went to the wake of a friend's uncle a few years ago (incidentally straight from work, without the opportunity to change due to having to catch a train to get there). I was dressed appropriately for a normal wake, as were about half the people. The other half - many family - were wearing jean shorts and Hawaiian shirts. That had been the departed's favorite outfit, and it turns out it was somewhat of a family joke. Apparently it hadn't been planned either, people just thought it would be a way to honor him.
I also recently went to the wake of a guy who my husband knew. The guy was in his late 40's/early 50's, a dad, normal professional job, little league coach etc and plenty of the people there were people who knew him from that part of his life and were dressed in normal wake appropriate attire. Then there were A LOT of folks in jeans and boots and band t-shirts: the departed had played in a band for 25 years, less often as he got older, but at least a few shows a year in local bars. His music scene friends came out in t-shirts from his band, or from shows they gone to with him that were meaningful memories of him. Plenty of his co-workers, and parent friends and neighbors didn't really know his band side and probably thought people were crass, but actually they were honoring him.