The rule of thumb here is whether the traveler is able to convince the Immigration officer that she is definitely planning to leave the country within the required term. A return ticket may be a factor, but it is neither a guarantee to entry nor it is required.
As a citizen of Ukraine, once I've traveled to the States holding a B-1 visa, and my return ticket had an open return date. So formally, I did not have ticket to a specific date. However, traveling for business purposes often has a good, documented, time-specific reason. In my case, it was my participation in an expo, and, obviously, an expo ends at a certain date. In my case, the officer looked at the expo invitation (two weeks or so since the date of entry) and stamped me 3 months
which I have used shamelessly.
Also, remember that the visa does not guarantee the entry to the country. Everything is up to the immigration officer. Let me put it even in a less neutral way: her entry may depend on a bad mood of a particular Immigration officer. So, don't set yourself up into a potential trouble.
So, your mother-in-law may be asked various questions so that the officer were sure she's not a potential offender of immigration rules.
What you may do is:
Try purchasing an open-date return ticket. This alone will simplify things alot (however, it might be more expensive than a fixed-date one).
Prepare your answers in advance. Remember the factors that may help your argument:
- Is it her first travel to U.S.? — the more times she has been there, the fewer questions she will be asked;
- Did she have prior issues in U.S., even driving tickets? — counter-intuitively, this may count;
- She's probably going to a certain event, doesn't she?
- Does she have an invitation letter from the company who invites her? — this may help, provided that the letter states some required time-frame for her stay in the country;
- It is also important to show how tightly she's linked to her (Ukrainian!) employer, family, and so on;