This thread makes me feel uncomfortable.
I'm sorry for your loss, first and foremost. However, do you think your grief could be clouding the issue? Anger at losing your mum seeking a place to land?
At the end of the day, your DW went to a hen with your permission. Hens by their very nature can include events leaning to the bawdy sexual persuasion. I agree with pp that for a hen do, the nudity can be more amusing than sexy.
She drew a naked penis and had a laugh while doing so. Unless she made sexual contact with the model, I don't think you have anything to worry or be cross about. I also highly doubt the model was "getting his jollies".
I also think you have gone slightly ott in the hoping no one finds out. It's a hen do and a live drawing class.
Also, you can't really control what other hen guests wish to post on the Internet. It's a pretty common thing to do nowadays.
To summarise:
The hen do
Your mum and your wife's relationship
The sad passing of your mum
Are all unrelated, but you are convoluting them in your head, and should try to untangle it all.
Again, sorry for your loss.