If your parents are like me, and want to know every possible detail in advance, you can share this with them about tipping: There is a farewell dinner at which the Adventure "wraps up." This is when many people choose to tip the guides. Over the course of the evening, people will discretely approach each guide and thank them and slip them an envelope. Of course, this is not a rule, but just what generally happens.
In the Adventure Handbook they will get in the mail shortly before their trip, there is a section with tipping guidelines, and there is a pocket in the handbook that contains, among other items, a couple of envelopes they can use for these tips. I've always brought my own note cards with me, and on the last night, before we go down to dinner, the three of us compose a personal thank you note to each guide and then I put the tip in that. I have always figured out the tip ahead of time (I go for the middle of the $7-9 per guest per guide per day, and round up to an even bill) and bring that cash on the trip so I don't have to worry about finding an ATM on the last day. (I just keep it in a separate envelope in the inside zip pocket of my purse so I don't spend it!)
I have to say that on our most recent
ABD, to Italy, we looked at that pile of cash, which seemed like a lot when I withdrew it from the bank at home, and added every US dollar any of us had left in our wallets. Our guides were just so over-the-top amazing that we didn't want to stay within the suggested range.
If they don't want to carry all that cash, the guides are great about pointing out ATMs on these trips and will happily accept Euros too. Your parents would just need to make sure their withdrawal limits will accommodate the equivalent of about $300, or remember to spread it out over a couple of days as the trip winds down.
