There is currently no ground to deny someone a visa or deny someone entry based on having used Medicaid. There is a ground inadmissibility if someone is "likely to become a public charge", but the determination of that does not consider non-cash benefits like Medicaid (except Medicaid for long-term institutionalized care). See this USCIS page as well as this section of the Foreign Affairs Manual.
She was denied entry for failure to overcome the presumption of immigrant intent. The presumption of immigrant intent applies to most types of nonimmigrant visas, and it says that the officer must presume the person is an immigrant, and thus ineligible for the nonimmigrant visa, unless they can prove otherwise. This is a generic reason used to deny visas when the officer feels that there is any factor in the applicant's situation they do not like (but there is not necessarily another ground of inadmissibility that applies), because, the theory goes, the negative factor makes it harder to convince the officer that you don't intend to immigrate. This ground of inadmissibility doesn't apply to immigrant visas, so you don't have to worry about that.
There is a proposed rule to change the way that inadmissibility for being "likely to become a public charge" is determined, and this could affect her ability to get a visa or to enter the US. One of the changes is that it considers more types of "public benefits" than before, including Medicaid, which is considered a "heavily weighed negative factor" if used for more than 12 months cumulatively in the previous 36-month period. However, the consideration of additional benefits is not retroactive -- non-cash benefits that were not considered under the previous rules, including Medicaid, are not considered under the new rule as long as they are not used or received after 60 days after the final rule is published. A final rule has not yet been published, so Medicaid she may have received in the past would not be considered even if she doesn't immigrate until after this new rule takes effect.
See the section "(f) Previously Excluded Benefits":
DHS would not consider public benefits under the proposed 8 CFR
212.21(b) that were previously excluded under the 1999 Interim Field Guidance if received before effective date of the final rule.
And the proposed 8 CFR 212.22(d):
(d) Benefits received before [DATE 60 DAYS FROM DATE OF PUBLICATION
OF THE FINAL RULE]. For purposes of this regulation, DHS will
consider as a negative factor any amount of cash assistance for income
maintenance, including Supplemental Security Income (SSI), Temporary
Assistance for Needy Families (TANF), State and local cash assistance
programs that provide benefits for income maintenance (often called
“General Assistance” programs), and programs (including Medicaid)
supporting aliens who are institutionalized for long-term care,
received, or certified for receipt, before [DATE 60 DAYS FROM DATE OF
PUBLICATION OF THE FINAL RULE], as provided under the 1999 Interim
Field Guidance , also known as the 1999 Field Guidance on
Deportability and Inadmissibility on Public Charge Grounds. DHS does
not consider any other public benefits received, or certified for
receipt, before such date.
Some of the other changes in the rule include mandatory consideration of factors like age (below 18 or above 61 is a negative factor), health, family status, financial resources (household income should be above 125% of poverty level), and education and skills. If she were to immigrate after the new rule takes effect, she would be subject to these considerations in determining whether she is inadmissible for likely to become a public charge. However, she is probably in working age, so that's not going to be an issue. And if your income is above 125% of poverty level (which you need to in order to sponsor her, unless there is a joint sponsor), that means her household income is too, and so that shouldn't be a problem either.
