John, assuming your friend is what the mainstream industry considers to be Plus Size and is of Agency standard all the main ones now have at least a small section devoted to what is delicately called Curve, with BOSS (Manchester) and MODELS1 (London) both having Boards catering specifically for this sector.
If she is not industry Plus Size, UGLY.org is worth a look.
Many thanks Bob for replying to my post. I will have a look at the websites you have suggested.She is an attractive girl facially so I don't think she would be in the UGLY category. She is very busty but with a bit of a tummy. I am really not sure how to advise her to be honest