My mom currently holds a valid B-2 Visa, which expires 06/25/2019. I bought her a ticket and she entered on June 3rd and her return flight is on the 28th. I read that she could enter the US any day until her visa expiration date. Now I’m checking her I-94 and her “Admit until date” is on the 25th (same as visa expiration date) so I know I’m confused wether she can still leave on the 28th and if that would affect her trying to renew her visa in the future since it’s literally impossible right now buy a return ticket to Venezuela before the 25th.
(This was originally a comment but I am turning it into an answer at the suggestion of @R..)
If your mother applies for a visa extension before her visa expires, she is able to stay while she awaits the decision for "a period of up to 240 days".
She can file online here https://my.uscis.gov/exploremyoptions/extend_non_immigrant_stay_us for a fee which is currently $370, probably less than it would cost to buy an emergency flight from the country.
Given decisions on extensions are currently taking weeks to months it is almost certain that she will not get a decision before she leaves in a few days. Particularly if she files such that it arrives just before her stay ends.
Please note that it is extremely important that she keeps a copy of the application for extension as well as the receipt she applied as this will act as her evidence that she did not overstay. The USCIS must receive the application to extend her stay before her current stay expires or this method will not work.
If the extension is somehow rejected before she has left, she will generally be given a 30 day grace period to vacate the country, which should still be long enough for her to catch her original flight. So whatever happens she should not have to change her travel plans. Note she should document everything thoroughly, this should not effect her ability to enter the US or get visas in future but might if she does not have all the documentation showing exactly what she did when she makes future visa applications.
If your mother's Admit Until date is 25th June then she must leave by that date, unless she can get an extension. If she stays beyond that date then her visa is automatically cancelled and getting another will be difficult. It also rules out any extensions. Source
The USCIS can grant extensions provided your mother meets certain criteria - specifically, she must not have overstayed (Details), but they recommend applying at least 45 days before an existing arrangement expires. Clearly time is short for your mother, so she'd better crack on with it if she goes this route.
You say that it's impossible to get a flight to Venezuela in time. Your mother doesn't have to go direct. As long as she leaves the US she'll be fine, even if that means taking a flight to Mexico or Canada or...wherever, and connecting from there.