I'm not forcing anyone to reply. Surely given the expected weather and outdoor setting then there are better settings than others ie shutter speed, f stop and such.
I am merely asking ppl with a far better knowledge of this than my girlfriend has so she can have some good photos to look at in future
I don't think you really realise the complexity though....
I expect tomorrow will be beautiful sunshine? If that's the case, then you have some interesting issues even if you stick the camera on AV mode at f8 and let it do the rest. Hell, even on full auto it's going to struggle. You know why? Because beautiful sunshine is horrific to shoot in often.
Shoot with the sun facing the people? Pretty solid lighting on the subject, faces well lit, camera will (probably) correctly expose. Sounds good? Yeah, until you realise that everyone in your shot is unfortunately squinting because the sun is in people's face...
Shoot with the sun behind people? Well, now people aren't squinting, much better faces. But now, the camera is probably going to expose for the scene, and there will be shadows cast across the people's faces. You have two options going forward, either stick in some compensating exposure. Faces are now correctly exposed, but your background is going to probably be significantly overexposed, which given the question, I suspect you wouldn't necessarily know how to correct in LR (especially as I'm thinking you won't be shooting in RAW). This leaves the option to use a flash as a fill flash for the faces. This sounds great, but I suspect given the power of the flash on the 1100d, it's not really going to be able to work fantastically in the level of sunlight we have at the moment. Even if it does fire and correctly light up the faces you have the final issue of the flash probably causing a bit too much harshness in terms of lighting. This can be vastly unflattering.
I had great fun trying to shoot in the same situation over the past week. It's taught me a lot about what I can and can't get away with, and plenty of the shots aren't as good as I'd like because of it.
If she wants to delve into manual mode then the best way to learn is to do. You need a rough understanding of App/Shutterspeed/ISO etc... But once you do, just go out and practice, at events exactly like this BBQ! IT will teach her so much. Thankfully we're in digital, so if she tries to take a picture of someone, looks at it, and it doesn't work, she can dial in adjusted settings and see how it turns out.
What's more, the best thing about the Nifty Fifty is that it can do wide apertures at a budget price. This is fantastic for creating gorgeous depth of field, but to do so, you need to shoot at these wider appertures. There's little point turning up with a prime lens and shooting at f8. You can do that on your zoom that has much more versatility for getting shots, and lots less footwork needed.
The best advice I can give mind given the short time period, is ISO 100, F2.8, AV. Good luck.
kd