I haven't had an etiquette question in a while, so thank you.
You make your friends sound pretty cheap for not chipping in towards the tax and tip. By the way, the patron is not charged tax on a comped meal in Nevada, so this must have taken place in another state. However, you should have made clear your expectations at the time of the invitation. So, I see fault both ways.
This at least makes for a teachable moment. I have been a part of comped meals in Vegas at least 100 times, sometimes when I provide the comp and sometimes when someone else does. The etiquette I follow, which is usually agreed to, is the one with the comp explains any restrictions on the maximum comp value at the time of the invitation. Then, everyone who was invited pays any portion beyond the maximum comp and the tip.
This question was asked and discussed in my forum at Wizard of Vegas.