If she definitely had a season a month ago, then things should be okay......except..... I had a bitch once who went into season so we put her in kennels as we had an entire male, the plan was to breed them but girlie wasn't old enough. she came home and 4 weeks later came into season again and the first I knew of it was when i found them tied!!!! I called the vet and she said that the season was unlikely to be a fertile one as there wouldn't have been time for the body to make more eggs, and she was right.
If the westie is humping everything in sight and he is adult, then I would certainly have him done before a bitch bites him...but I know he's not your dog.