South facing window sill is good, as long as it had time to acclimate to more sun. It's not in optimal soil, and it would be best to have it in a pot with a drainage hole, unless one was drilled into the jar. But then you don't really want to go and repot a friend's cactus, especially if you're new to this, so you've got kind of a conundrum there. Do you know how long your friend has had it? If anything bad happens to it, you're not really responsible, so I would try not to obsess too much over it. All you can really do is give it only enough water so that it goes to the bottom, and make sure the soil gets dry between waterings.

It looks like a member of the genus Coryphantha - not a commonly found genus. It would definitely benefit from a repot into a gritty mix in a pot with a drainage hole. But how much do you want to get involved in your friend´s plant.

Try cutting the soil with perlite and placing it in a terracota pot.

Glass jar with no drainage is not good as far as cactus are concerned. Get a plastic or terra-cotta pot same size as the jar but with a hole in the bottom. Make sure it gets plenty of sun. You can transition it slowly into some full sun each day, but don’t do it in that glass jar as the roots will get too hot.

Check with your friend first if you feel uneasy about repotting his or her plant. But it could die under those conditions of a glass jar.

It might just be the picture or a reflection, but the soil in the jar looks green about half way down. If so, that is probably algae growth. It can happen in a jar or glass container with no drainage. This plant was in a plastic pot with lots of drainage holes and I sure didn't expect to see this happen. This is what I am afraid will happen in that "pot."

See the mix that my plant is in? That is 50/50 mix of cactus and succulent soil and perlite. If you have regular potting soil on hand you can use it and add 60% perlite to 40% soil. I am making the assumption that this plant will stay indoors. That mix in a clay pot should work fine for your friend's cactus. Please don't add any sand to the mix and don't put rocks in the bottom of the pot. Neither of these things are good at helping with drainage - in spite of how many times you see it on the Internet. LOL

The reason my plant had the algae -- I am in SW Florida in high humidity and lots of summer rain. My plants are outdoors and that mix just can't dry out well in the summers. So I had to go to a more complicated mix - and that just means it was a bit of a chore to find the components to my mix.

Ask more questions if you need more answers! Good luck!