It very much depends on the situation of the individual concerned. Whilst there is a logical basis for assuming why many single parents may be under greater financial pressure in general, it is not true that every single parent is struggling and likewise there are many couples that also are.
In simplistic terms i imagine the relative financial difference between 2 parties is the most important factor.So,for instance, if i wanted to borrow money from a friend or family member i would only feel right about asking those i felt or percieved to be in a stronger or more stable financial situation than myself (remembering that my perceptions may not be entirely accurate) whether they happened to be a single mother or not.
So,is it right to ask a single mother for money? Under certain circumstances it might be.
Is it appropriate to go ahead and ask someone for money when you know they would be adversely affected by loaning you said money ? Generally, i wouldn't think so but there could be exceptions depending on the relative situations of the person asking and the consequences of them not recieving versus the consequences for the individual being asked for the money, were they to hand it over.