Most English departments are not seeking an expert in Latin, manuscript studies, multiple medieval languages, etc.
You're right about this, snowbound, and it's why I've been careful to state throughout this thread that my own experience is at a research university.
However, looking at this year's medieval advertisements for English, I can see that many different types of institutions say they are looking for a medievalist who can effectively cover both Old and Middle English, as well history of the language. This latter requirement in particular isn't negligible-- the ability to teach the history course properly would usually be connected to advanced, technical training.
In other words, there are non-research positions being advertised right now, whose requirements would not be easily filled by someone without the kind of training you seem to think would be necessary only for research-oriented positions. That training might come via a medieval studies program, or via a high-end single-discipline program, but the point is that what to a non-medievalist might seem esoteric (philology, paleography, linguistics, manuscript studies etc) is often in fact central to ANY future teaching in the field, and some of the skills are more readily acquired in "studies" programs.
Your advice to the OP's friend about making a choice of field, however, is spot on. I would never advise anyone to enter any program (interdisciplinary or otherwise) without a pretty clear sense of where s/he hoped to be, 5-10 years down the road.