I've done it, and she loved it

I've used many variations:
- Thumb in her vagina and middle (or index) finger doing her ass
- Index and (/or) middle finger in her vagina and thumb in the anal
- A scissor like thing: Index finger in her vagina and middle finger in ass (hand vertically, like you would be shaking hands..) This one can also be varied many ways; Two fingers in vagina and one in anal. I've also used my thumb to massage her clit. If his fingers bend to this (wasn't too hard for me) it's a killer combination

Remember him to keep his fingernails short and smooth (to prevent cuts) and also to never go straight from anal to vagina. I mean he shouldn't put a finger that's been in anal in the vagina unless it's been carefully washed in between.
Mr. G