From what you state, you are using your friend's ownership benefits as a guest. As a guest you are limited in receiving benefits but you have zero responsibility. IMO, before you contemplate anything that incurs expense ask yourself some questions and do the research. For example. use this forum and others like Red Weeks and other TS owner forums that rent TS to compile a list of rental costs incurred at the locations where you are already familiar with. Compare the rentals to what it would cost you to modify the existing deed(s) and pay continuing maintenance fees (MFs).
If your friend wants no part in retaining ownership you could purchase for as little as one dollar. Your friend would then have no further responsibility. On the other hand, you could become a co-owner in which case either of you individually would be responsible for paying MFs for as long as the ownership lasts. This could be in perpetuity or might be time-limited. The current ownership documents spell this out. Regardless, you will need to engage a real-estate attorney, familiar with the contracts unique to TS properties. Not all real estate attorneys are.
What are the MFs? Historically, they go up about 5% annually. In 2022 with the acquisition of Diamond by Hilton, the MFs will be 9% higher. Also, Diamond has a history of higher MF compared to some other TS companies. Finally, if the property is at greater risk than average of incurring damage due to acts of nature, is old and in need of repair, remodeling, updating, etc, owners could be subject to assessments. Assessments are over and above MFs.
Next, check out a couple of vacation clubs and compare costs for the places you are familiar with - Are the costs Less? Equal? More?
Essentially, you are interested in getting the best benefit for the least cost - as well as the least responsibility.
Years ago, before the internet and vacation clubs and discounted travel and lodging promotions, the options available to vacationers with / without families to rent apartments (not hotel rooms) was rentals from owners. This is no longer the case.
Finally, I just attended an "owners" update at a Diamond property. many of the details critically important for owners regarding the aforementioned merger are not know yet. That said, I was told that in 2022, a Vacation Club will be launched. Second, the current tiered ownership (standard, Silver, Gold, etc) currently in the Diamond system will be replaced simply by ownership benefits commensurate with points in possession allocated. Fixed week owners are not affected by this. Points will be good for 5 years once allocated. Points can be used to pay for a variety of fees at the value of $0.30 per point and the type of benefit and accumulation of such is tied to the number of points. Points allocated as a result of a purchase contract have an associated MF. For example: I was shown a proposal to purchase 6,000 points at a cost of $10.00 per point with an annual MF of $1,500.00. Imagine that ! $60,000.00. Of course, This is a proposal. Much like a MSRP, the cost os negotiated, but even at a 50% reduction, this is a sizeable cost. Keep in mind, every "owner update" is associated with a pitch to purchase more points to "get you to get more benefits."
I realize this is a lot of information and I apologize. However, you will find that this forum is full of dedicated and knowledgable people willinf to help one another in getting the most out of the TS industry.