How is the 18th Arr in Paris?

It is safe. That is the area of Montmarte and where the Sacre Coeur and the Moulin Rouge is located.
Where exactly?

There are great variations in the 18th, a big arrondissement. An address is necessary to give clear advice. $100 is 88 Euros and change. Is that per night for the four of you?

Does the ad show a 13 digit City registration number for the rental? If it is a legal rental it has one.

The arrondissements are huge and varied. We have stayed in the 18th for a month at a time several times west of Sacre Coeur on Rue Caulaincourt and also briefly on Marcadet and really enjoyed it. Some areas are very touristy and some are quiet and residential and some are vibrant with large African and middle eastern immigrants -- and their fabulous food and fabrics etc. Whether young women would feel comfortable will depend a lot on where they are exactly and 88 Euro is pretty low for an apartment for 3 people. The touristy parts of Montmartre are not cheap.

The location is boulevard Ornano

Yes, the Price is per night for 5 adults.

Area is mostly residential but pretty far out. Depends on your closest #4 metro stop. Simplon is better than Marcadet or Clignancourt. You're not within walking distance of anything much except the gorgeous Art Deco Henry Sauvage designed housing complex and swimming pool which is probably of little interest to tourists. Buses change routes the 20th of April but it looks like the #80 from the 18th Marie at Jules Joffrin will keep its route to points in the 7th and the #4 metro is good to central Paris. Is being this far out worth the savings on an apartment? Does this distance work with your goal of really exploring Paris? I don't know what the area is like at night.

€88 for 5 people per night is €17.60 per person.

I don't know how you can have "cute" at that price anywhere in Paris.

I would look, look, look and read every single word in the description and lower pages, and read carefully every single word in every review. I hope that there are pages and pages of AirBnB reviews to show it has been around for a while - even that it really exists - I would ask Google where the photos come from, and I would check that mandatory registration number (13 digits, starting with 95), and I would check carefully the neighbourhood with Street View.

I learned a long time ago that if something seems too good to be true there is a chance that it is.

How much are the extra fees? Cleaning can sometimes be charged into the €€€ range and make sure that buried in the lower pages there isn't a high extra person charge.

Don't deal with the vendor direct - use the AirBnB communication and payment channels. Being requested to come "off-line" and deal direct is a big red flag, as is a request to wire funds or use Western Union before they arrive.

Or it may be just fine and a great holiday. I just want them to remember the experience for all the right reasons.