I've had lots of cavities filled. Never had laughing gas for a filling - but it will definitely help relax her.
Tell her it is NOT going to hurt. Her tooth may be a little sensitive to hot and cold for a few days after the filling is done, but it's a breeze. The noise of the drill is the scariest part - and noise doesn't hurt.
With the laughing gas, which isn't dangerous, it'll relax her and make her feel kind of like everything's far away, but she'll still be aware of what's going on, so that's not scary.
Then they'll rub something on the gum to numb it, so she definitely won't feel any pain when he gives the shot for the novacaine. She'll feel the pressure of the needle, but it won't hurt at all.
They'll use a drill to "clean out" any rotten part of the tooth or teeth (I'm not sure what inbetween two molars means - whether it's just one tooth with a cavity or both teeth). They'll clean it, and then pack whatever filling stuff they use.
That's it! Whatever part of her face that is on will be "numb" from the novacaine for another 1/2 hour or hour or so. It means she won't have muscle control in that part of her cheek and side of her lips, so if she tries to drink or something, use a straw, or some water or whatever will dribble out - we always thought it was kind of fun and funny.
The most important thing is to make sure she understands it won't hurt. She may feel some pressure from the needle for the novacaine, and she'll be able to feel the dentist moving things around in her mouth - but she won't feel any pain.
And while there may be people that have reactions to laughing gas or novacaine, the numbers must be very, very small and generally very, very safe because it is such standard stuff used by all dentists, orthodontists, periodontists and oral surgeons.
In fact - if you're used to having cavities filled, it's pretty easy to skip everything and just get the shot of novocaine. My mom used to skip even that.
Best of luck - I'm sure everything will go well!