Do you have to become a Monk to attain enlightment? It seems that the answer is no, but in this case, why would you want to become a monk instead of staying "normal" ?
A lay person can realize Nibbana in a very short period of time(In few years or even in a shorter period of time) If s/he practise mindfulness all day long with effort. Even a very young lay person can realize Nibbana. Realizing Nibbana isn't about being in a particular place but it is the result of the right living. Not all of the people who realize Nibbana becomes a monk because some of the once-returners becomes spiritual teachers or they are in an old age and they are retired from their jobs etc. 

In most cases being a monk is absolutely better than being a lay person because human world is crazy and is opposite of spirituality. The human civilization is the manifestation of the narcissistic human world's madness. But a person's decision to become a monk depends on many factors. If a spiritual person has a sangha and in an old age and retired maybe s/he would not want to leave the lay life and become a monk. A person who realized Nibbana and especially an once-returner is in continous bliss and contentment so s/he would not leave his home If the conditions are suitable for him/her.(Solitude or having a sangha etc.) 

So a lay person's giving up the spiritual practise is a wrong idea because realizing Nibbana in the lay life is certainly possible.