Khalwah if Another Woman is Present
Question

                My wife told me that a gas canister ran out at home and has to be changed, but I am not at home. Can a worker come to install the gas canisters at home when I am not present? Please note that there is another sister who will be with my wife in the house so as to avoid leaving her on her own with the worker.
Praise be to Allah.
    Firstly: 

    The scholars (may Allah have mercy on them) are unanimously 
    agreed that it is haram (impermissible) to be alone with a non-mahram (a 
    woman whom one is allowed to marry according to Islamic law) woman 
    (khalwah), because of the saheeh hadeeth (authentic narration): “No man 
    should be alone with a woman unless there is a mahram (male relative whom 
    she could never marry) with her.” Narrated by al-Bukhari, 5233; Muslim, 
    1341. 

    An-Nawawi (may Allah have mercy on him) said: If a non-mahram 
    man is alone with a non-mahram woman, with no third person present, this is 
    haraam according to scholarly consensus.

    End quote from Sharh Saheeh Muslim, 9/109 

    As-San‘ani (may Allah have mercy on him) said: This hadeeth 
    indicates that it is haram to be alone with a non-mahram woman, and there is 
    consensus on this point.

    End quote from Subul as-Salam. 1/608 

    Secondly: 

    There is nothing wrong with a woman allowing a non-mahram man 
    to enter her husband’s house when he is absent, as long as two conditions 
    are met:

    1.     Her husband should have given her permission to do so.

    2.     There should be no khalwah (i.e., she should not be 
    alone with him); rather there should be a mahram with her such as her father 
    or brother, or anyone for whom it is permanently forbidden to marry her. 

    If she does not have a mahram with her, but there is another 
    woman with her, khalwah may be avoided thereby, but it is not permissible 
    for a woman to put herself in such a situation unless the woman is 
    trustworthy and the man who is with them in that place is also trustworthy 
    and there is no fear that he could overpower the two women. 

    An-Nawawi (may Allah have mercy on him) said: If a non-mahram 
    man is alone with a non-mahram woman, without a third person being present 
    with them, this is haram according to scholarly consensus; this is different 
    from if a man is with a number of non-mahram women, in which case the 
    correct view is that it is permissible.

    End quote from Sharh Saheeh Muslim, 9/109 

    As-San‘ani (may Allah have mercy on him) said: It says in the 
    hadeeth that “the third one present with them is the Shaytan”. Can someone 
    other than the mahram take his place in that regard, so that there will be 
    someone with them in order that khalwah will be avoided? It seems to be the 
    case that he may take his place, because the reason for the prohibition is 
    the fear that the Shaytan may cause temptation between them. 

    End quote from Subul as-Salam, 1/608 

    Based on this, there is nothing wrong with the man allowing a 
    non-mahram to enter his house, on condition that he is not alone with his 
    wife and that he is trustworthy. However, the basic principle, and the best 
    precaution to safeguard religious commitment, is that he should not allow 
    this unless there is a mahram present. 

    And Allah knows best.