LOL, maybe it's just my skewed view from the horses I get and see, but when I hear "Oh, this horse hasn't been ridden in a year or so, could you do a tune up on him?" it generally means "This horse is a rodeo bronc that has bucked off every cowboy I know and he hurt me bad last time I tried to ride him, so I turned him out and haven't touched him since".
When I bought my horse he hasn't been ridden for quite a while (supposedly a year but I believe longer) and the latter was exactly the reason his previous owners stated, though the truth was very different as I learned by time. When I first asked about that thin orange horse, they told me he's a visious man eating beast that just loves injuring people. Except he was never turned out but locked in a 3,5 m x 4 m stall... Anyway, when I finally got him I started lunging him, took him for a few walks (not riding him), spent a lot of time with him, I just wanted to learn about his temperament, things he has problems with, things he is comfortable with etc. I rode him once or twice a week for two months or so so that he slowly gets used to the routine and starts developing some muscles and soon I was able to ride him eveyday without a single problem. He was scared of the lunge rope, didn't know how to canter, what a sitting trot was and everytime I tried to apply any leg pressure he would freak out but never I had a problem with him being nasty, he just simply had no idea what I was asking him to do. He is the sweetest, most willing and people loving horse I've ever worked with, it turned out his previous owners had absolutely no idea what to do with a horse so they were basically just scaring the heck out of each other. He still has a lot of issues but today I can put a baby on his back without being worried.
I would definitely not just start riding him as if nothing has happened, you don't know his past and how he will react. It's definitelly better to start slowly and then once you finally hop on him realise he has no problems at all and maybe regret you didn't simply hop on him at the beginning than just jump on him out of nothing, being tossed around and having much more to fix in the end
That's just my opinion and what worked for me though.