Frisian is still the most flourishing regional language in the Netherlands, and has retained a certain status in small parts of Germany and Denmark as well. If you look
here, however, you'll get an idea of just how early Frisian was supplanted in various areas. In East Frisia, it was the Saxons that supplanted the Frisians. If you look at East Frisia, you'll see most of it stopped being really Frisian in the period 1300-1600. Saxons moved in. I'd actually argue that if Frisia is absorbed into some proto-Dutch polity from the outset, and if this is governed like a confederation of provinces much as the Dutch lands have historically been... then this might aid the Frisians. A big point would be to avoid combining the Frisian coast and the Saxon hinterland into single provinces. If you keep the Frisian provinces apart from the Saxon ones, that improves the chances of Frisian to thrive as a language. (Observe that all areas where Frisian came to be a language governed from a Saxon-speaking city -- such as Groningen -- Frisian declines, but where it was governed by Frisians themselves, it continued to thrive.)
If you can make sure that the city of Groningen becomes part of Drenthe, whereas the (initially Frisian) 'Ommeland' remains a Frisian polity, then you get a contiguous Frisian area that includes all of East Frisia. If you can manage to have this area be united as one big Frisian province within a Dutch context, you may be quite confident that Frisian will remain the primary language there until c. 1900 (assuming roughly the same kind of history as in OTL), and will remain a strong regional language thereafter. (Basically, you'd be setting Frisian up to be as successful in the Ommeland and in East Frisia as it has been in OTL Dutch Frisia. Considering that OTL Frisia is pretty much bilingual, that's a pretty cool outcome for the Frisians!)