I'm 47 and I lift 5 to 6 days a week with the 6th day not being heavy or rigorous. I wouldn't do less than 5 on or off cycle.

3 days a week is pretty much a rule of thumb as the minimum. I would encourage that to anyone.
Now, Im gonna step off the cliff and on to a few toes here but it is what it is.
Yes! He would build muscle from 600mg Sust a week on a regular diet and working out once a week, although not to the extent of someone who was busting their balls at the gym and eating 3500 clean calories a day. It WILL put him in a positive nitrogen balance and he would build muscle. Its why we give anabolics to cancer and AIDs patients. It helps even if they don't get off the couch, or if they don't have their diet dialed in.
BUT let me add he would do so at his detriment. See busting your ass at the gym, being well hydrated and eating clean actually helps to reduce the worse side effects of injectable steroids which are bad lipid profiles and polycythemia. Both of which will kill you dead. So is it worth the trade?
Hope that helps clear things up a little.

I'm 43 and I only lift three days a week. Then do light cardio on non weight days. But I also spearfish for a living. So there's no such thing as an off day really. Mentzer for life baby! Brief, intense, and infrequent! And I'm still growing!

I'm 45 I go 6 times a week once a week will not work you need the pump, Aldo diet must be in check.

Have you asked him why he is only lifting once per week? I'm 48 years old and my body does not recover the way it did 25 or hell even 5 years ago. If he's lifting that much weight I'm guessing he still has some quality LH going on that are keeping his test levels up and in good shape. If its recovery issues he may be better off checking out his hgh levels and whether these are in decline causing recovery issues. This is a much more difficult testing because the body only secretes it at certain times. Normally in the middle of the night when in deep sleep. A quality hgh regimen may be much more useful to this individual than getting on test because he already has good test levels likely. He needs bloods to ensure but at that sorta decline weight I feel confident its good but a cheap test result will tell the tale.

He threw in the max on the decline!!!!!! Classic.... I might favorite this for just for that line....

What is his goal for the cycle?
Lifting 1x/wk isn't going to cut it.

There are guys who get away with three times a week, but they are monsters in the gym. Once a week doesn't make sense and would be a waste. There is no way you can do an adequate whole body workout in one day and then have 6 days off and make any progress.
A range would be 3-6 days a week.
Also, sust isn't a good first cycle. Should stick with test-e or c and 500 mg a week or less for the first go around.