@marcopolonian she's not actually forcing him to switch sides. Actually, if you read the previous volume omake, you'd remember that there was two main factions among the Hussites. The "moderates", led by the nobles and higher society of Prague, the so-called "Utraquists", and the "radicals", led by Zelivsky and Prokop, the so-called "Taborites", who want a total revolution.
What she's hinting at is that the Utraquists, led by Cenek of Wartenberg, are negotiating with Emperor Sigismund because they are afraid of the Crusade and in order to be allowed to keep part of their faith (the 4 articles of Prague) and because they believe that the taborites are doomed. Sigismund just wants the crown of Bohemia and may leave some room to the Utraquists to practice their faith, because they are nobles, like him, but won't negotiate with the radicals.
She's telling him that the situation of the taborites is hopeless, that they are going to be crushed by the crusade, while the nobles are ready to cave in to Sigismund. She's enticing him to join the nobles sides and use his troops and his gifts of commander against the Ottomans and if he gets enough achievements, they may obtain more freedom of cult. Everyone wins (except the radicals).