While it might be best practice to use a trade mark as an adjective to maintain distinctiveness and avoid a mark becoming generic, it does seem rather unlikely that most people will refer to Apple Watches as Apple Watch devices. But it’s understandable why companies are careful. No one wants their valuable marks to go the way of ‘aspirin’ in the UK and the US and careful use of marks can be essential to avoid them becoming generic. Apple’s guidelines on using their trade marks state that trademarks are adjectives used to modify nouns; the noun is the generic name of a product or service and as adjectives, trademarks may not be used in the plural or possessive form.
If your friends have Watches or, as Apple insists, ‘Apple Watch devices’, pluralisation of the sacred trade mark being prohibited you can send them a brief recording of your heartbeat, played back haptically, or a little sketch made with your finger, or an audio message.