If the time for one of the tefillos (e.g. shacharis, mincha, and maariv) arrives, and one doesn’t wish to daven right away, I have heard that one may delay the particular tefillah if he set an alarm to remind him to daven at a certain time. But if one does not have access to an alarm, may he ask another person to remind him to daven?
Yes, one may ask someone else to remind them to daven. In fact this is the preferred way to do it, and this way is mentioned in the Mishna Berura 235-18.
It should be noted that this applies to davening Mincha and Maariv, however before shacharis we may not do these things even if we have a shomer to remind us. The reason for this is because we should be involved with our creator before getting involved in our own business.
Binyan Oam 1-1, Shvus Yitzchok 2 pg. 287, See Ishei Yisroel 13-31 and footnote 90. Regarding using an alarm clock as a shomer to wake someone up, it is quoted in the name of R’ S. Z. Auerbach zt”l that he held that an alarm clock should not be used to wake someone up, because he may shut off the alarm and then fall back asleep.