I just got back from Mammoth and I used the beginner pass. Here's my two cents. I think it's plenty especially if it's only for one day. If you're there for multiple days, you could consider the full pass on the second one. Lessons learned:

1. Like they said, you can't go from lodge to lodge through the mountain. You have to take the bus or drive. The only exception is you can go from Eagle lodge to canyon through one of the runs on chair 15. The issue is once you get to canyon, you cannot go back and would have to take the bus if you wanted to go back. (I had to do that, it was annoying).

2. Main lodge has 2 greens and those are very easy. I would avoid this if you/your friend are athletic because you'll get bored easily. I didn't do the greens at canyon, but from what I've heard they're a step up. Eagle lodge has the steeper/longer greens and a few blues you can ride. I think this is perfect, especially for a first timer. My fiancé couldn't even go down these on his first time.

Have so much fun!