Etiquette-wise, some people feel that texting condolences or any serious news is inappropriate, and would prefer a call rather than a text, while others may feel texting is an acceptable gesture of sympathy. Like some other etiquette rules, it depends on personal view.
Nevertheless, a card or letter may be the most appropriate approach, as the receiver of the condolences can hold on to the memory in a keepsake box or book. However, people under the age of 20 may find it acceptable to send serious news, such as condolences, via text, whereas older age groups may view texts as disrespectful and impersonal. As more and more teenagers become adults, texting or emailing may become the more acceptable method of expressing condolences.