Fr. Matthew was a well-known and regarded Coptic Orthodox monastic and abbot of the famous monastery of St. Makarios the Great in the Scetis desert. He reposed in communion with the Church, and was only ever officially censured for various ecclesial-political issues. Nevertheless, before his repose there was an ongoing conflict with HH Pope Shenouda III regarding various theological intricacies.
In my opinion, the conflict was fuelled by a misunderstanding stemming from a difference in theological education and outlook. Pope Shenouda seems to have a prima facie suspicion towards language speaking about our progress towards God-ness, or becoming like God. I think this suspicion results primarily from a mixture of a) a lack of patristic grounding, and b) sensitivity towards his Arabic cultural context where such language and concepts are prone to being misunderstood. Fr. Matthew on the other hand is very well-rooted in the patristic mind (and I think his work Orthodox Prayer Life attests to that quite well), and theologises strictly on patristic terms*.
All in all, for whatever my opinion, as an OO, is worth to you, I certainly found nothing unOrthodox about Fr. Matthew's Orthodox Prayer Life. On the contrary, I believe it to be one of the most valuable insights to Orthodox Spirituality, not only account of its evidencing a striking familiarity with the patristic mind, but more importantly on account of the fact it is grounded in the experience of one who lived the life of prayer in solitude for over half a century in one of the most ancient and blessed monasteries of the early Church.
*HH Pope Shenouda's stance in this regard does not seem to coincide with what appears to the be the generally unanimous approach amongst key contemporary OO theologians and heirarchs. Strong language of deification is especially present in the works of Fr. T. Malaty (who is probably the most renowned Coptic theologian amongst lay Copts, given the prolificacy of his works, and his ability to make theology accessible to the laity) and the great Paulos Mar Gregorios (who taught the profound maxim, "deification is humanisation", in his Cosmic Man). My Bishop's work on Icons (HG Bishop Daniel of Sydney and its Affiliated Regions, The Spirituality of Icons) refers to the notion of Icons witnessing to the concept of deification; and a prominent priest in our diocese, who is also a key lecturer at our theological college, Fr. Matthew Attia, briefly discusses deification as a blessing of the Incarnation in his work Blessings Upon Blessings.