It is permissible for a woman to travel without a mahram provided the way, destination and return journey are safe and provided she does not encounter any harassment jeopardizing both her safety and religion. It has been narrated through 'Uday Ibn Hatem (may Allah be pleased with him) that the Prophet told him, "And if you live a long life, you will surely see women traveling from Hira till they circumambulate the Ka'ba, fearing no one except Allah" [Bukhari and others]. Imam Ahmed's report of the hadith includes: "By He in whose hands is my soul, verily Allah will bring this matter [Islam] into completion till women travel from Hira and circumambulate the Ka'ba without being accompanied by anyone."
From this hadith with its different chains of narration, some scholars have derived the permissibility of a woman traveling alone if her safety is guaranteed. Moreover, they have used this hadith to restrict other prohibiting hadiths which, according to them, referred to the lack of security associated with travel in the distant past.
The majority of scholars have permitted a woman to travel for obligatory hajj without a mahram if accompanied by trustworthy females or company. They based their opinion on the precedence of the Mothers of Believers (may Allah be pleased with them) who went on hajj after the Prophet's death and during the caliphate of 'Umar (may Allah be pleased with him) accompanied by 'Uthman ibn 'Affan (may Allah be pleased with him).
Imam Abu al-Hasan Ibn Battal mentioned in Sharh Bukhari (vol. 4, p. 532), "[Imams] Malik, al-Awza'i and al-Shafi'i said, 'A woman who does not have a mahram, may travel for her obligatory hajj with other women in trustworthy company; the majority of scholars permit this. Ibn 'Umar accompanied women from among his neighbors for hajj. This was the opinion of 'Ata`, Sa'id Ibn Jubair, Ibn Sirin and al-Hasan al-Basri who said that a Muslim man other than her mahram may even be more protective."
The opinion appropriate for fatwa at present is that it is permissible and there is no objection to a woman traveling alone by the various safe routes and means of travel via their venues such as airports, harbors and the like. This applies whether she is traveling for something obligatory, recommended or permissible. The hadiths forbidding a woman to travel without a mahram pertain to lack of security which was the case in previous times. Based on this, if a woman's safety is ensured, the prohibition is lifted.
Imam Al-Baji, the Maliki scholar, mentioned in Al-Muntaqa Sharh Al-Muwatta` (vol. 3, p. 95) that a woman's travel without a mahram is contingent upon the number of people accompanying her. It is impermissible for her to travel alone if she is with a small group of people. However, it is permissible for her to travel alone without a mahram if she is accompanied by a large number of people who guarantee her safety. This opinion was reported from al-Awza'i.
The luminary and Maliki scholar, Al-Hattab, mentioned in Mawahib Al-Jalil Sharh Mukhtasar Khalil (vol. 2, p. 523) that al-Baji restricted the permissibility of a woman traveling alone without a mahram in the case of a small group. Quoting Al-Baji, al-Hattab said that it is impermissible for a woman to travel alone if she is with a small group of people. However, it is permissible for her to travel alone without a mahram if she is accompanied by a large number of people who guarantee her safety. This opinion was reported in Al-Ikmal whose author did not mention any contesting opinions. It was likewise mentioned by al-Zanati in Sharh Al-Risala as the established opinion of the Maliki School, thereby restricting the opinions mentioned in Musannaf Khalil and other books.
Al-Zanati said, "It is agreed by the Malikis, including Malik himself, that it is permissible for a woman to travel without a mahram, whether for obligatory, recommended or permissible matters, if she is accompanied by a large group of reliable people, whose number is such that it is certain that she will be safe on her way and at her destination."
Based on this, there is no objection to your wife traveling alone if her safety while traveling is guaranteed.