My first suggestion is that getting advise from strangers over the internet is not a good idea in this situation. Too many man-hating feminists, divorce lawyers, and people with agendas on one side or another.
She should talk to her friends and family, somone who knows both of them and ask them their opinion of her boyfriend, their opnion on whether or not she should marry him, and their opinion on whether she should try someone else.
My advice (reread paragraph 1 please) is that she should make a decion about whether or not to marry the guy. Trying to "see other people" and then marry him is a poor idea I think; what if she decides he is the best thing ever, but he has found another woman? Or if she finds someone very charming who turns out to be a snake --- she now has the same decision; should she try again or go back to the first boyfriend. How much experience is "enough"?
Excellent advice for any situation.