Question: It is said that we should not be attached to emptiness…
Answer: … and not be attached to form too, while it is practically safer to be skilfully ‘attached’ to pure forms (such as that of Pure Land) as the best stepping stone first, than to be unskilfully attached to emptiness when the Middle Path is not realised yet.
Question: How does one become attached to emptiness when it is empty?
Answer: (Well, how does one become attached to form when it is empty too?) Attachment to emptiness means to not relate productively to (and with) forms, but to cling to thinking of them being impermanent and unsubstantial in a pointlessly nihilistic way. This is to forget that impermanence and insubstantiality of phenomena mean all can be directed to change for the better too.
Question: As a deceased friend has passed on, so has his form. There should be letting go of him emotionally, by accepting this as a form of emptiness. Should emptiness be thought of this way?
Answer: This should be done after his death: http://purelanders.com/2016/07/19/what-should-be-chanted-during-after-a-funeral The physical form is not ‘a form of emptiness’, as the Heart Sutra teaches that, ‘form is the SAME as emptiness’ – even before his passing, even when alive, and even after his passing, even now.
Rather than thinking of his form being empty now, (which though true, it was also empty when he was alive,) you should think of him still undergoing constant change now, (as before,) while what you do in the link above can help him to change for the better. The true consolation comes not from thinking of his form as empty only, but from knowing you are doing your best to still help him, without clinging to form or emptiness.
Clinging to form as in missing him physically does not make sense as his previous form has changed much since. Clinging to emptiness as in the wrong extreme thinking that he is totally ‘no more’ does not make sense as though changed, his changing state still can be helped. Functioning between is the Middle Path.