Wemmick knows Pip's house will be closely watched, so he goes to Herbert for help in finding safe lodging for Provis. The fact that Compeyson is in London means they must be very careful where they hide Provis. When Herbert finds a room in the house where his fiancee lives, Wemmick feels this is a great choice. Its location near the river will give Pip a better chance of getting Provis on a ship leaving the country. Pip has no reason to go to Herbert's fiancee's house, so if he's being followed, Pip wouldn't lead Compeyson to Provis. Herbert can keep an eye on Provis, and Herbert's fiancee can always send for Herbert if she notices anything out of the ordinary.