Effectively having an exisiting loan impacts the amount you will get as a mortgage on the basis of repayment afforability. For example, with the 10k loan, your girlfriend has financial commitments of (say) 250 pm. The banks tend to operate on the basis of a max of 40% of disp income allowed to serrvice loans (mortgage and any other financial commitments). Thus if this loan were cleared, you would qualify for a mortgage with payments 250 pm greater than the one you would get without clearing the loan. So you can work out backwards what difference that could make to the capital sum available to you.
As shoppergal said, a few grand can make a massive difference to your mortgage approval limit...