It is like this, if he has seen a few cubensis, wild or domesticated, he can't miss them, they are obvious, especially if he IS in florida and the like. Cubensis are one of those more distinct species that are pretty unmistakbable. but if he HAD seen them before, then he wouldn't be posting pics asking what they were. I've never seen grey gilled cubensis, but he says they print brown purple - and they bruise blue, so he has a pretty good chance.
But now you have to worry about ecoli from the crap they were taken from. One is always wise to simply take a nibble when they think they are sure, it won't always help, and it may screw with your trip, but it is a good idea.
There are many many mushrooms that both don't taste good AND are not poisonous. I really don't know how many poisonous ones taste good. I also am quite certain that a kingdom that is just an assortment of very involved organic chemical factories that are capable of producing antibacterials - all the way up to very powerful psychoactives can also come up with some pretty elaborate poisons - imagine.
Beyond EVEN that, there are poisons that only become poisonous in the presence of other chemicals. There is at least one species that is very nice, until you decide you might like a glass of wine with your mushroom soup. - I did that and was rather unpleasntly surprised and very glad I didn't have more than a glass of wine. luckily I knew what I had done (sort of, it's a long story).
There are other mushrooms that are perfectly fine - after being cooked.
And, now here is the most interesting thing of all - Agaracine - is carcinogenic. Now agarcine is contained wthin the agaricus bisporus that you put on your pizza, the common button mushroom is without a doubt carcinogenic, cooked or not.
but, it is always a good practice to do exactly what I said.