MWO is only online multiplayer, no single player at all. There is a tutorial and training grounds without multiplayer, but there also isn't any actual game in those either. He can do solo drops in the public solo queue, which will place him in a team with 11 other solos against 12 other solos. However, to get the most out of it, he should join a unit and drop in the group queue or community warfare. Voice comms is highly recommended for group play, in addition to the in game VoIP. Solo pug dropping is still fun, but group drops are generally better.

Community warfare is better left until he has a good stable of fully elited/mastered mechs with modules. Until then, solo and group public drops will be the main drops.

To get that stable of mechs will require a lot of grinding, real money, or both. The big concerns for free players are c-bill rewards (in game free currency) and mechbays need to elite chassis. Using consumibles without premium time really slows down the rate you earn c-bills, leading some to call it "pay to win". However, consumibles aren't really necessary. For mechbays, you get three free, but must either pay for more, win them in events, or earn a few in community warfare. The issue for free players is that it takes three variants of a mech chassis to elite the chassis and unlock skill boxes to improve the mech. Having only three makes it rather hard to build up 4 elited mechs for community warfare, much less 4 mastered mechs.

All mechs in game are available without unlocking, just need the c-bills or real money. Several trial mechs are available for use for free, but you can't customize or unlock skills on them until you buy one. All mech chassis are also available with the free currency, but there are hero variants that require real money. The heroes aren't overpowered, and some are actually underpowered. Most of the top tier variants are available for c-bills instead of requiring real money.

Joystick support is a bit spotty. Some die hard joystick players have got them working, but really the keyboard and mouse are the better choices.

There is also a large learning curve. However, having played previous MW titles, it should be easier for him.

Since it is free to try it, might as well set him up for a go. The combat is fun.