A couple of buddies of mine have created a really cool product called the BagTowel and have created a
KickStarter project to get the word out and raise money to expand their business.
The towels are really cool as they are partly made from recycled sailboat sails. The towels are very large and very comfortable. If you spend anytime at the beach, lake, river or pool, you'll want to get your hands on one. They are also great for taking to the gym as the pocket is perfect for your toiletries and shaving supplies.
Give their kickstart page a look -
https://www.kickstarter.com/projects/1031947627/the-bagtowel-by-urban-barrels

Here's a short commercial they put together for the BagTowel. What do you think?
Cool. Good luck to your friend.
The wife and I always end up picking up one of the cheap nylon versions of this when we go to a theme park or various events. Just handy to have.
I might have to grab one of your friends version. Never thought of using it for a gym bag.