I use what's recommended. I think 5.5bar in my 32mm and 8.5bar in my 23mm.
I guess your bike is much more comfortable than hers since you have less pressure. This is something you have to weigh-up before you change tyres. What do you want...less resistance or comfort? Getting both might be difficult.
If you want to roll with more ease, get some smaller profile slicks. Are you wife's wheels the same size? Ie. 26 or 700.