I have a service person that repairs DATs but honestly the D7 is such a low level consumer product that it would not be worth opening up.
I transfer DATs and cassettes as a side hustle and I use only Tascams, which are professional units and parts are readily available, although it is often the case that I have to use a couple broken ones to cannibalize parts. I have four Tascams recording to Tascam DR680s, then transfer the SD Card data to a hard drive.
Edit: I'd just throw the D7 away if I was giving advice to your friend, and buy a decent professional DAT like a Tascam like a DA20 or DA30. They can be had for $300-500 off Craigslist and eBay and often are in decent shape, but then again, one could buy it and it could crap out in a month and require servicing, belts, and the like. Buying 20 year old machines is a crap shoot. That is why I've got a half dozen and four working machines. Let me know if you need the guy that services these--he works for the major studios in Hollywood and is very well known in the recording industry. Its about $150 to open the unit up and service it. If one is doing a couple hundred DATs, that is a small price to pay. My DA20 has over 500 DATs under its belt (no pun intended) without service, and I picked it up from a stereophile for $500.