2230, Diphenhydramine, and a Hospital Heliport

No. Two independent disqualifiers, plus a regulatory one.

• Medication. The alcohol-and-drugs rule bars any crew member from acting while using any drug that affects their faculties in any way contrary to safety. Diphenhydramine is a sedating antihistamine: drowsiness, slowed reaction time, and degraded attention are documented effects, and they are additive with fatigue. 'I took it hours ago' and 'I feel fine' are not the test - subjective alertness is a poor detector of impairment, and the impairment often appears after the point where you would need it most.
• Fatigue. Awake since 0515 and on your feet all day puts you at 17+ hours of wakefulness at launch, in the dark, doing a task that requires precise altitude and position control near a hospital heliport. Fatigue degrades vigilance and error-checking in the same way the medication does.
• Night operations additionally require the completed night training and functioning anti-collision lighting visible for at least 3 statute miles - you have both, but compliance with the lighting rule does not make an impaired pilot legal.

The pressure pattern here is the tell: a same-day paid job, aircraft already loaded, and a plausible-sounding justification. That is a hazardous attitude, not an operational analysis.

The rule is that you may not operate in the vicinity of an airport in a manner that interferes with operations or the traffic pattern. A hospital heliport is an airport for this purpose, and it is charted.

• Monitor and announce. Use the published CTAF for the heliport: state your location, altitude, and intentions (for example, a rooftop survey at 150–200 ft AGL, 0.6 SM from the heliport, beginning in 5 minutes and lasting 20 minutes).
• Stay out of the pattern and the departure path. Establish which way the helicopter departs and keep your aircraft clear of that corridor. A 1,500 ft climb-out crosses the altitudes you intend to use.
• Yield, don't share. If the helicopter is departing toward your site, hold on the ground or relocate until it is clear. Rotorcraft at night are hard to judge for distance and closure, and you are the smaller, less visible participant.
• See-and-avoid at night: keep the strobe running, avoid pointing any light at the helicopter, and use the observer to acquire and track it. Do not rely on hearing it - the aircraft is louder than the helicopter at 100 ft.
• Deconflict the return leg too: the helicopter's arrival later in your flight is a second conflict you should plan for, not react to.

Decline tonight and offer a specific alternative.

• Say it plainly and briefly: 'I can't fly tonight - I've taken a medication that isn't compatible with operating, and after a full day I'm not in a condition to fly at night near a hospital.' You do not owe a client a medical file, but you do owe them a clear reason rather than a vague excuse they will try to talk you out of.
• Offer a real option: a morning slot (better light for a roof scan anyway), or a rebooking with a specific date and time.
• Do not hand the job to someone unqualified. If another certificated crew is used, they must independently meet the night requirements and be fit to fly - you cannot pass along your own compliance.
• Document it: log the declined flight, the reason category, and the reschedule. If the client later disputes a delay, the record answers it.

The underlying judgment: the aircraft being in the car, the calm air, and a working strobe are all true and all irrelevant. Fitness of the crew is a preflight item, and it failed before the aircraft was ever unpacked.