Disclaimer: I worked directly with dentists who were entrepreneurs and "invested in real estate". Sometimes they have a singular belief that they themselves can't move past (MB2 canals all should be referenced to endo only comes to mind). This doctor's thinking is very short-sighted because it's what they are comfortable with, but it may not fit your goals. In fact, I see a lot of disadvantages to investing in condos and my buy box is almost exclusively that I don't buy it if I don't own the entire roof. Someone else's negligence becomes your problem, associations are often poorly run, and assessments can hit hard out of nowhere.
As for living in Manhattan or another high cost area, unless you plan to go into fee only practices, do high end cosmetic work, etc, your ROI on that expensive dental education isn't great. Insurance payouts are decreasing and overhead is increasing, so it sounds like this is ego driven bad advice on multiple fronts.
I have seen significant appreciation in the suburbs, and they are often less likely to have significant value swings like you have in downtown areas. My Downtown Los Angeles condo value has swung several hundred thousand in value either way in the 2 years renting it. I've never heard of it taking two years to sell a house in the suburbs, that's straight idiocy.
Going forward ask this guy about treatment plans, what CEs to pursue for after you graduate to help make you a better clinician, etc., but his real estate advice sounds like a wanna-be ego-driven baller who wants "sexy" properties to brag about at a dinner party with people they don't actually like instead of educating themselves about how to build a long term empire so they don't even have to bend over that perio patient in their op 8 hours a day. I'm not mincing words because I dealt with a lot of these types previously. They'll continue to gripe about all the hours they put into a career they no longer want, but didn't look for the actual exit door opportunities.
Happy to chat real estate or DAT and dental school prep/ future of dentistry with you anytime!