Been there, done that many times, though a bit smaller...
... and I did so when I had a smaller boat. However, their are some adjustments in thinking. Some of the addaptations and thinking apply equally to folks with artritis (my wife has had knee replacment), so this is not just a weight thing.
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I wouldn't worry about the FRP structure. Unless they are on-deck running about, the loads are less than good size blokes jumping and falling in rough weather. Pick a calm day and there should be no trouble. Big folks tend to sit in one place.
Boarding will be a problem. Specifically:
* The long step (tie the boat tight to the piling during boarding). VERY important. I have a small hand tackle specifically for this. Also damn handy when loading heavy provisions. Add an extra fender rather than get offended if something rubs.
* The use of railings (if they pull hard there could be damage). Tying, described above, really helps. Offer a hand (and don't allow refusal). This also depends on the railing design.
* Dock height. You may need a step up or down.
Steps will be a problem:
* The companionway may be impossible and probably dangerous if heeling.
* Make them aware the head may be inaccessible. They need to know, if this is true.
* The steps down into the cockpit may be VERY difficult without a hand hold, since they are 2x height and there is no railing. Moving the boom to a good location and hanging a loop of webbing from it may help. Also consider if a box for a step is needed or would help (something VERY stable and strong--the helm foot rest in this post is very strong, has rubber on the bottom, and is used for this:
http://sail-delmarva.blogspot.com/2009/05/handfull-of-minor-pdq-upgrades.html).
PFDs: All kidding aside, they won't sink.. if they know how to float. However, the horse shoe-type should adapt. I would make sure they can swim well. If not, I might use that as grounds for refusal. I hate taking non-swimmers.
Re-boarding: make sure they understand that there is no practical provision. They are over the rated load on the ladder, and it's worse when the boat is moving. They will have to stay in the cockpit.
Clothing: If rainsuits are needed (spray) it is very unlikely they have these.
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Of course, chosing to maintain a weekend hide-away is entirely your business; there is nothing wrong with alone time. I cherish both family and the absense of family.