Short answer: It is best to avoid coffee during Śvetāmbara Paryuṣaṇ.

Reason and practice:

• Paryuṣaṇ is for saiyam (self‑restraint) and apramāda (vigilance). Stimulants like tea/coffee are considered mādak dravya (exciting/stimulating), so they are discouraged.
• If you are fasting (upavās, attham, etc.), only boiled water at allowed times is permitted; tea/coffee is not allowed and would break the fast.
• If you are doing ekāsan/biyāsan (one/two simple meals before sunset), traditional maryādā still recommends giving up tea/coffee as a special ras-tyāg (renunciation of a taste) for the festival.
• After sunset, no eating or drinking (other than what your specific vow permits), so coffee is certainly not to be taken then.

Note on traditions:

• You asked about Śvetāmbara practice; the above reflects that maryādā. Digambar practice is generally even stricter about beverages during fasting.

If you have a health condition, take guidance from your guru and physician, but as a religious observance, avoiding coffee during Paryuṣaṇ is the proper way.