The thing she was suspected of can literally be as small as simply giving him an alibi
Essentially, they suspected her of believing that a crime had been committed and failing to tell someone
Obviously people have opinions on how good of a person you may be if you give someone an alibi or something of the like, but I think it's simple to say 'oh in her place I wouldn't have done it' when the reality is that you don't really know if you would have, or how their relationship worked
Some other people have said elsewhere that it would have probably been semi-normal for him to be home a bit later sometimes, to look a bit more stressed on some days and the like