Double check that the problem isn't actually your hearing aid's Bluetooth codec support, rather than the physical driver's supported frequency range.

Bluetooth audio devices need to agree on an audio compression format before they can send audio, and the default lowest common denominator codec that all audio devices support is extremely bad at low frequencies (and is all around terrible). It could be that your hearing aid is good enough but doesn't support whatever high-fidelity codec your coworker's stethoscope supported. Investigate what codecs your hearing aid supports and what the stethoscope supports. You'll probably want something like LDAC, LC3 (aka LE audio), aptx HD, aptx adaptive, or preferably even aptx lossless, to be supported on both devices.

If you're considering investing a lot of money anyways, you may want to consider replacing your hearing aids with ones that support this functionality (the frequency response and the codecs).

As someone else already pointed out, if your current hearing aids don't support below 100hz, then even if you play the sound over a speaker, your existing hearing aids are just going to act as earplugs. You may need to invest in different hearing aids or just live with taking out your hearing aids for the foreseeable future. Or I suppose just blasting the heartbeat over the speaker to bypass your hearing aid (caution wrt audio feedback lol)