The show can make up whatever rules it likes, of course, but given the current ratings/public perception it does need to be careful how it treats this.
The least controversial idea would be to have no replacement, as it wouldn't be entirely fair to have someone enter now when there are only 6 others left. That would leave a bit of a headache over how to fill the remaining 6 weeks with just 6 contestants.
Favouring one single person would give the impression of too much meddling by producers, and degrade the contest further as a result. The only exception would be the reinstatement of the most recently removed contestant (Johnny) with the slightly stretched narrative that Frankie's rule break occurred prior to the weekend etc and that he cost Johnny his place in the final 7. Bearing in mind the show was keen to get rid of Johnny, it's very unlikely.
Beyond that, some kind of public vote to determine the replacement might be a more likely scenario...that way they could spin the story that the replacement has done something to earn their place in the final seven.