Short answer: It is best to avoid coffee during Paryushan.

Why:

• Paryushan is for maximum ahimsa, saiyam, and aparigraha. Coffee/tea are stimulants and habit-forming, so renouncing them supports saiyam and ras-tyag.
• Many households follow boiled-water-only rules (water boiled before sunrise). Preparing coffee later adds taste and involves extra handling, increasing subtle himsa.
• In common Shvetambar practice during Paryushan, tea/coffee is usually taken as a “tyag.” In Digambar observance (Dasalakshan), with ekasana/ayambil and boiled water discipline, tea/coffee is also generally refrained from.

If there is a genuine medical need:

• Take it only as aushadhi (medicine), in the smallest necessary quantity, using pre-sunrise boiled water, without embellishments (no sugar, flavors), and with your guru/elder’s guidance.

Best practice:

• Keep to plain, pre-sunrise boiled water.
• If you lapse, accept it, do pratikraman, and resume the niyam.