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Christians make a lot of mistakes and I have made a fair few in my time. I am guessing here but their concern may be with regard to fornication which is a sin in the Bible. Now if you look at this Wikepiedia page it says that fornication is a euphemism for prostitution so your mum is safe there.
http://en.wikipedia.org/wiki/Fornication
Then I looked up the Greek and I found that fornication means to be a harlot (prostitute) and includes adultery, incest and idolatry. Seeing none of those things are happening I can't see where your mum is doing anything wrong.
G4202
πορνεία
porneia
por-ni'-ah
From G4203; harlotry meaning prostitution, adultery, incest and idolatry. Isia.1.
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