In the UK a mortgage offer in principle, which it sounds like you have, is normally valid for 90 days. Check with the lender, but you'll find this is the norm due to fluctuating interest rates.
In terms of your credit file, every time you apply for any credit, loans, store cards, overdraft, mortgage etc etc, you leave whats called a footprint on you credit file. 1 or 2 footprints isn't really a problem, but if a lender sees that you've applied for loads of credit in a short space of time, they are less likely to offer you credit.
So technically your colleague is right, but for the purposes of the mortgage, if the lender says yes today, they're unlikely to turn you down in 4 months on the back of their own credit search.
Hope this helps.