Hi Sue,
Well I'm going to have to disagree with Natasha here. (Sorry Natasha!
) Mexico is a big country and as in the US there are places that are safer than others and I believe it is perfectly safe to rent a car and drive all over this part of Mexico. In fact, if you are doing a lot of travelling it's the only way that makes sense. You don't have to figure out when (or if) buses are going where, and you can lock you belongings in the trunk if you want to wander around the ruins and such!
Both times I have been to the Yucatan we rented a car and put several hundred miles on it through our wanderings. In fact the only time I've been scared in traffic was when I was riding in a cab!!!
You will want to purchase insurance though (in fact most places it's mandatory to purchase liability insurance, and for extra you can get collision insurance too). But in my opinion it's the easiest way to get around if you want to see a lot of different places. If you can change a tire you shouldn't have any problems.
(Just buy a good map before you go.)
I bought a guidebook that should you decide to go to Mexico I would suggest getting. I looked at several, bought two and this is my favorite. It's called "Yucatan & Southern Mexico" by Cadogan. It's very informative giving history on the area, places to stay, general prices, where to eat, things to do, where to buy gas, how to get around, etc., etc., etc.
You didn't say whether your friend was male or female, so in case it's a she I want to quote a bit from the book: "On the whole women travellers do not experience great difficulties in southern Mexico. . . the pervasive machismo archetypally associated with Mexico is far less noticeable in the south . . . Two women travelling together will rarely encounter harassment" In fact, I was going to go see all the ruins with just my sister, but then she couldn't come so I convinced my husband to come with.
My only concern with your trip is the time you are going. It's going to be getting pretty HOT! We got tired of drinking warm water so for this trip we brought powdered Gatorade with us. It makes warm (and even very warm) water much more tolerable!
When I read about Xcalak in my guide book I wanted to go, bad! But we already had our trip planned out for this year so I'll have to save it for another! Belize is not far from there either and it would be easy to get a bus to Belize from Chetumal. You won't be able to drive a rented car there. However, if you have a rental car you can park it for about $7 - $10 a day in a 24 hour protected lot. We do that in Playa del Carmen when we go stay in Cozumel for a couple of days. Then when we come back from Cozumel we just grab our car and drive to the airport. Sure beats trying to haul all your luggage on a bus or into a taxi!
Anyway, I could go on and on about this subject so feel free to ask me any further questions you might have.
Good luck, Beth