She may have a difficulty of getting a visit visa. Her being the mother of a British child may give the UKBA an idea that she might not go back to the Philippines when her visit visa runs out. I mean they might think that if your nephew suddenly decides to stay permanently in the UK with the kid, she has a big probability of staying here permanently as well even if her visa constricts it. So it is always better to go for a settlement (fiancee) visa with their situation.