Sorry,this is probably my first post on here after lurking a long time. I have been around forever on BB and AM. I am 67 yr old and have been lifting since '51 when my dad put some poles, paint cans and cement together to make my first bbell. My normal body weight used to be around 175 because of my mountain biking and running (I lived much of my life in EUrope and Asia). I arrived back in the US in '00 a malnourished 149 lber, going through divorce, maximally depressed and completely fallen out of shape.
Today I am 5'10 3/4 ", 215 lb who has recently done 380 on the Hammer Strength BP. I'll try to get a video up of my recent Triceps Pressdown attempt. My path the last 8 years had the advantage of lots of muscle memory and a lifetime of discipline and training. Still I would make some of the following suggestions for your 70 year old newbie.
Slow start and use very, very light machine exercises in a fullbody, 3X per week workout program. Get him started walking on his days off easy and build up slowly. In 6 months or so you should be able with slow increases in each form of exercise, be able to get him into a split routine and maybe start some HIIT cardio approach with him. I recommend the use of machines, no matter how light at first to ease him and guide him through the learning and muscle reactivation phase.
Importantly, IMO for us old farts is to feel a bit tangible accomplishment even if it is going from 25 to 50 lbs on a machine BP eg. It is part of a very necessary feedback loop that reenforces a tangible image of progress which will keep him coming back. And that is every bit as important as getting him started. With a base you will be able to then really guide him into a mixture including some free weights with your knowledge and expertise. By then with your diet and supplmentation advice he should be hooked and cooking on his own motivation.
Apropos the diet portion, consider his health needs and get him to make good use of say oatmeal, olive oil, green veggies heavy with broccoli sprouts etc. This is keeping an eye on the health needs of we elderly while supplementing the typical healthy diet I am sure you know better than I. On the supplement side there are a few that really stand out if he is able to swing it.
Omega 3 fish oils (eg for me 5 gms ED)are extremely helpful for his joints while adding a world of assistance to a wide range of health issues especially important for us such as Alzheimers, Prostate Cancer, Heart and Cholesterol problems etc. Likewise I am a believer in heavy use of Vitamin D3 which has proven benefits again for we oldsters in training while fighting Cancer, Cholesterol etc. Green Tea Extract will give him some energy to work out while addressing some 25+ health situations.
Lastly I believe strongly in having him use Creatine (your choice, I find I react best to CEE but that's just me) and that is not only for strength and recovery issues but its now famous use in preventing Parkinson's disease (Already in 3rd stage testing with HIH).
I stack Beta-Alanine along with my Creatine (it's anothe great old farts supp) because it teams well for recovery and fights off the effect of lactic acid while being arguably the hottest element in eye care as it combines with our Histidine to form Carnosine and fight off Cataracts and ARMD (Age Related Macro Degeneration).
I apologize for my long winded reply, but I needed to introduce myself a bit and perhaps help you understand that I am not a newbie myself but a life long, and I do mean long LOL, iron lover. Get his guy off and running and you may really keep him running for many more years in optimum health. Viel Glueck!