My wife has been a nida for several weeks after she had some issues post breastfeeding. She is traveling during chol hamoed for work where there won’t be a mikvah anywhere near her. The next time she will be able to go will be once the second days of pesach start and it will be embarrassing since her whole family will know.
Can she go tonight on day 5 of clean days instead?
No, as difficult the situation may be there is no room for leniency with regards to the seven clean days, and immersing in the mikvah before their completion does not remove the nidda status.
In terms of your situation, a few things to consider: double check mikva availability sometimes there are mikvaos available in surprising places. If it is impossible to go without someone knowing, sometimes by confiding in one person and getting their help it can be hidden from everyone else. If this is not an option, and husband and wife are agreeable, the mikva ay be pushed off a day or 2 to avoid embarrassment.