Okay endometriosis aside, I beg to differ!
At the moment, her inability to conceive is unexplained. If she has age on her side, then there is time to investigate and hopefully resolve the problem whilst she is relatively young and subject to the same chances of conception as anyone her age ( e.g 25 year olds have 1 in 4 chance of conceiving every month), In these circumstances I would agree that it isn't unreasonable to wait 2 years before investigation. However, if she is between 35 and 40, her chances of conceiving every month are 1 in 7 or 8. Add this to a late diagnosis of a fertility problem - whatever the nature - and long waiting lists for treatment, then it might be too late.
My point is, if she is older and is forced to wait 2 years for investigations and these reveal a problem, she may be wishing she tried sooner in life.
Can you see my point?