residence permit and visa

Hello, I have a question about travelling out of Belgium without a residence permit.
My girlfriend is from Ecuador and she's studying in Leuven, she arrived 4 months ago, she asked for a residence permit but it haven't arrived to her house yet. The problem is that she's planning to travel to London this saturday but she's worried that she could have some problem when she returns to Belgium only with her visa and not with a residence permit or id card? Is it possible to have problems because of that or her 1 year visa is enough to enter to Belgium?