I suffer from anxiety (along with depression and low self esteem), so I do know how easy it is for a worry to get its claws into you.
For months I have been worrying about how I would cope when ds3 went off to university, because then all three boys would be living away from home either permanently (ds1 graduated this year and is working hundreds of miles away) or during university term time. Dh works away from home a lot - his job changed a year ago, and he was moved to a different arm of the company so, instead of having an office half an hour away in Glasgow, his office is now on the Edgeware Road in London - so he is away long days, and often a night or two a week.
I had myself entirely convinced that I would not be able to cope - that the nights, in particular, would be awful. I had a picture of myself lying awake every night, alone in the darkness, convinced I was entirely alone in the world. I was sure it would catapult me back into deep depression, and that basically my entire life would disintegrate.
I was wrong. The boys have headed off to university/work and I have had the first few nights when dh has been away - and I was fine.
The OP's problem is that the issue she has fixated on is not one she can easily find evidence to disprove, in the way I have disproved my worries. We can give her the logical arguments, about how careful this chap will be, and why - but she is still imagining microscopic contamination of her home, and that fear is stronger than the logic at the moment.
I agree with those who have said she needs to seek help and support to deal with her anxiety issues - even if she accepts the logical arguments on this thread, and lets them defeat this particular fear, if she hasn't dealt with her anxiety itself, she could just move on to the next fear.
I look back now, and see so many times when I have let a stupid thing build up into a huge monster under the bed - it is not a good way or a happy way to live. I hope that she can get the help she needs, and live a happier, less stressed life.