My condolences on losing your friend. Going from easiest to hardest. Next season and byeond you need a replacement owner, and finding one shouldn't be too hard. Don't sweat this.
Dealing with the remainder of this season is the challenge.
Finding a replacement owner works, and they may be even willing to stay in the league, but the dynamic changes. I'm sure someone out here could help you out as an interim if you want that. If you need another friend to take over, that may be tough, but you might find one.
If you don't want an interim, and/or want someone you know, but can't find anyone, you can freeze the roster, but if it's a head to head league or one where you have daily/weekly lineups that's a poor solution, as it means the team isn't remotely competitive.
However, if you have weekly starts, you can freeze the roster, and "automate" lineup selection by establishing rules which the team will pick starters - that won't be too bad. Fantasy points per game over a time period is a pretty easy one to apply, as you alter lineup when the best players by a criteria change. However, this doesn't handle injuries, etc, so you need rules to cover injured/suspended players as well. Easiest thing to do is if a player is suspended or on IR at time of roster selection, they get skipped over. Day-to-day injuries are rough for this sort of thing, and you need to pick whether to skip for those or not. I'd lean towards leaving those guys in the lineup, since it can be hard to get reliable info, and it's a gray area if the commish starts making decisions on who to start based on news, not an actual status.
In a daily start league I think you really need to go with a replacement owner.