You shouldn't keep that kind of stuff in its not healthy if you don't feel comfortable with him doing things for that woman and being 7 hours away, be honest with him and let him know how you truly feel. Although trust is a big thing in a relationship tell him it's not that you don't trust him but you know how being a newly widowed female is and then precede to tell him that when you first lost your husband you were lonely and vulnerable and you just don't think you can handle him helping her so much especially it being so far away, anything could happen. If he truly cares about you he will understand that you aren't being paranoid and that you just care about him and don't want to lose him to anyone or any reason.
There is nothing wrong with being completely honest about your feelings rather than keeping them in and cause yourself stress and anguish. If you shut your feelings down about something this minor could you imagine how it would be if you kept your feelings in for something more major it's just not healthy. Stresses can cause many health problems and this is why you should tell him exactly how you are feeling and don't hold back your words if you are scared tell him you are scared if you are feeling jealous tell him your feeling a little jealous, this will show him that you at least care about him enough not to lose him and maybe if he knows how you feel and you explain to him how you were when you first lost your husband maybe he could see signs if the woman does make moves on him.
Tell him face to face before he leaves to go to Monterey so that he will have that in his mind when he goes to help this woman with her house also tell him could he possibly work on fixing you home when he is finished with the friend's house. Make sure you are confrontational when you go to him and talk to him you don't want to have an argument before he leaves that wouldn't be good at all. If you need more help let me know or if you have something else to add or ask.