I have always thought the term 'PM' is wrong, and I prefer 'PNF'. 'PNF's' job is manyfold apart from 'monitoring' but he/she should remember that it may be a matter of seconds before he/she needs to become 'PF'. There is no real need for 2 crew to 'monitor' the A/P, 'PF' should be doing that as long as input and mode selection are checked, but when manual flight is happening, then the 'M' really begins.
Up to that point experience level is not that significant, and providing he/she is trained to a basic standard and imbued with company SOPs/procedures, 'PNF' should be more than capable for the task. With a new/inexperienced PNF it is a judgement call whether or not - and when - to 'inflict' A/P disconnect on them.